In certain areas in a fluid system, it can be necessary to provide an indication of the level of liquid within the system. More particularly, it can sometimes be necessary to provide an indication of the level of liquid within a tank, vessel, pipe, or conduit within the fluid system. By knowing the liquid level, it can be determined whether remote apparatus (alarms, pumps, valves, etc.) should be activated, e.g., to increase or decrease the level of liquid within the fluid system.
A conventional liquid level device, also known as a "sight gauge", "sight glass", or "armored liquid level gauge", includes a cover clamped to a chamber. A window interposed between the cover and chamber allows visual inspection of the level of liquid in the chamber. The gauge can be fastened, e.g., bolted or welded, directly to the wall of a tank or vessel, or connected in parallel piping to a pipe or conduit. Graduations on the outer surface of the liquid level gauge around the window provide for visual measurement of the level of liquid within the device, and hence within the tank, vessel, pipe, or conduit.
However, conventional liquid level devices can have certain limitations under corrosive, hazardous, or other harsh environmental conditions. In particular, the window in the device can become corroded, rusted, or otherwise obscured, which decreases visibility through the window and thereby increases the difficulty in accurately measuring the level of liquid. Accordingly, certain liquid level systems have been developed which are not dependent on the visual inspection of the liquid within a liquid level device.
For example, one type of liquid level system is shown in Holroyd, U.S. Pat. No. 5,015,995. Holroyd discloses a first pair of transducers mounted on opposite sides of a tank in a vertically upper horizontal plane, and a second pair of transducers mounted on opposite sides of the tank in a vertically lower horizontal plane. Pulse generators send electrical pulses to transmitter transducers which transmit stress waves into the wall of the tank. The stress waves propagate peripherally through the walls of the tank and are detected by receiver transducers. The receiver transducers are connected to processors to analyze the amplitude of the detected stress waves and to determine if liquid is present or absent at either the upper or lower horizontal plane. The processors can operate alarms or valves to control the liquid level in the tank.
Similarly, Freedman, et al, U.S. Pat. No. 3,456,715, discloses electro-acoustic transducers arranged on opposite sides of a mold for molten metal which transmit and receive radiation through the mold. The measurements obtained from the detector transducer are indicative of the amount of radiation absorbed by the molten metal between the source transducer and the detector transducer--the absorption increasing when the molten metal reaches the level of the path connecting the two transducers.
Another type of liquid level system is shown in Beard, et al, U.S. Pat. No. 2,960,678. Beard discloses an ultrasonic level measuring apparatus having a transmitter and receiver mounted on the top of a tank. The transmitter is designed to transmit mechanical wave energy downward into the tank, where it is reflected off the interface between liquid in the tank and the substance above the liquid, and collected by the receiver. The receiver is connected to a control valve which regulates the height of the liquid in the tank.
Still another type of liquid level system is shown in Vollhardt, U.S. Pat. No. 3,111,581. Vollhardt shows a water level gauge having a radioactive source and a detector. The radioactive source is included within a float which rises and falls with the level of the water in the gauge. A variable-thickness baffle interposed between the source and the detector provides for increased radioactive transmission as the water level, and hence the float, rise; and decreased radioactive transmission as the water level falls.
Similarly, Green, U.S. Pat. No. 2,676,265, discloses a radioactive source located within a tube connected in-line with a fluid tank. A detector mounted on the top of the tube detects radiation produced by the radiation source. The radiation is attenuated depending on the level of fluid in the tube, and hence provides an indication of the level of fluid within the tank.
Other types of liquid level systems include a float which is designed to rise and fall with the level of liquid within a tank. The float can provide a visual indication of the level of liquid within the tank, or can be coupled to additional circuitry for an electronic output. Alternatively, capacitive or thermo-resistive elements can be located within the tank to provide an electronic signal proportional to the level of fluid within the tank.
The above-described liquid level systems are useful in determining the level of liquid within a tank, vessel, pipe, or conduit. However, these systems can also have certain limitations. For example, some tanks must be drained and/or depressurized, and their operation interrupted while the liquid level system is installed (or adjustably moved, removed, serviced, and/or replaced); while others can require apertures be formed in the tank and that certain parts of the liquid level system (e.g., the transmitter and/or receiver) be welded to the tank. For tanks containing hazardous, toxic, or flammable fluids, the installation of the liquid level system can be extensive, costly, and time-consuming, and precautions must be employed to safely perform the modifications. Accordingly, some of these systems do not have the flexibility to be simply and efficiently mounted (or adjustably moved, removed, serviced, and/or replaced) on an existing tank, vessel, pipe, or conduit.
Further, it is believed that some of these systems, in particular the systems with acoustic or ultrasonic transducers, are susceptible to dust, dirt, and moisture which can make them unacceptable for harsh environmental conditions. Additionally, some systems must be acoustically coupled to the tank and can be dependent upon the particular liquid employed.
One known type of liquid level system which has overcome some of the above-described limitations provides transmitters and receivers using microwave signals to determine the liquid level in a tank. The microwave transmitters and receivers are non-contact and can transmit and receive microwave signals through sealed windows to determine the content height in the tank. The microwave signals are generally immune to airborne contaminants such as dust, dirt, and moisture and can operate through large thermal gradients.
The transmitter for this one type of microwave system typically includes an oscillator, a mixer diode and an antenna. A portion of the microwave signal generated by the oscillator is coupled to the mixer diode. The antenna focuses the remainder of the signal on the target, which reflects a portion of the signal to the mixer. During transmission, a frequency modulation is applied to the oscillator usually in a triangular, sawtooth, or sinusoidal fashion. The mixer then combines the source and return signals, generating an output frequency which is a function of the frequency difference between the two signals. The frequency relationship is, in turn, a function of the distance from the transmitter to the target. As the target moves with respect to the transmitter, the distance changes as well as the output from the mixer. One such system is manufactured by Saab Tank Control of Gothenburg, Sweden.
Although the Saab system illustrated above provides certain advantages for measuring the level of liquid within a large tank, the transmitter and receiver are relatively large, bulky, and expensive units which must be mounted within openings formed in the roof of the tank. Accordingly, the Saab system, like the other liquid level systems described above, requires that the integrity and operation of the tank be compromised. Additionally, it is believed that the bulkiness of the Saab transmitter/receiver combination prevents these units from being used with a liquid level device such as a liquid level gauge. Specifically, the liquid level gauge has a relatively small window area in which to direct and receive microwave signals, the surrounding metal housing of the liquid level gauge being unacceptable for microwave transmission. It is believed that the transmitter/receiver units of the Saab system are far too large and bulky to provide an accurate determination of the liquid level within the liquid level gauge.
All of the above-mentioned liquid level monitoring systems, and it is believed nearly all presently available liquid level monitoring systems, require expensive, costly, and time-consuming varying of the liquid level in the tank, vessel, pipe, or conduit to directly visually calibrate, test, and verify the functional operation of the liquid level monitoring system. Also, it is believed that such visual direct validation is normally accomplished by reading installed sight glasses or liquid level gauges.